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1.
Since the 1980s, one of the important progresses in the study of the Qinling orogenic belt is marked by findings of numerous ophiolite zones[1—4]. On the basis of the former orogenic models of the Paleozoic colli-sional orogeny[1,5,6] and the Mesozoic collision[7—9], another orogenic evolution model from the Paleozoic subduction-collision along the Shangdan suture to the Mesozoic final collision orogeny along the Mianle suture[3,10], including the relicts of the Jining orogeny, has been pr…  相似文献   

2.
The middle sector of the Yarlung Zangbo suture zone stretches over 200 km long from Ngamring through Geding to Rinbung, roughly along Yarlung Zangbo River valley (Fig. 1). This belt resulted from the closure of the Tethyan ocean and the collision be- tween Indian plate and Lhasa block[1―8]. Lots of works demonstrated that rifting of the Tethyan basin in southern Tibet started from Triassic time. Initial oce- anic crust appeared in the Late Jurassic, and then ex- perienced a rapid sprea…  相似文献   

3.
The Caledonian North Qilian orogenic belt lies between the North China plate and the Qaidam mi-croplates, and resulted from the collision among the Qaidam microplate, mid-Qilian block and the North China plate. The orogen initiated from the rifting of the Late Proterozoic Rodinia, and then it experi-enced stages of Cambrian rift basin and Ordovician archipelagic oceanic basin, and foreland basin during Silurian to Early-Middle Devonian. The average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn and Lan/Cen from cherts of Cambrian Heicigou Formation are 0.797, 0.627, 1.114, 0.994 and 1.034 re-spectively. In the NAS standardized REE distribution pattern, the cherts from Xiangqianshan is slightly HREE enriched, and the cherts from Ganluci and Shiqingdong are plane. All of these features indicated that Cambrian cherts of the Heicigou Formation originated from a continental margin rift background. On the contrary, the average ratios of Al/(Al Fe Mn), Al/(Al Fe), δ Ce, Lan/Ybn, Lan/Cen of the Ordovician chert from Dakecha, Cuijiadun, Shihuigou, Laohushan, Heicigou, Maomaoshan, Bianmagou, Da-chadaban, Baiquanmen, Jiugequan and Angzanggou, are respectively 0.72, 0.58, 0.99, 1.09 and 0.96 respectively. Their NAS standardized REE distribution patterns of most Ordovician cherts are plane mode or slightly HREE enriched. The REE distribution pattern of few samples of cherts are slightly LREE enriched. Characteristics of sedimentary geochemistry and tectonic evolution demonstrated that the Cambrian-Ordovician cherts, associated with rift, oceanic, island arc and back-arc volcanic rocks, was not formed in a typical abyssal oceanic basin or mid-oceanic ridge. On the contrary, they formed in a deepwater basin of continental margin or a archipelagic ocean tectonic setting. Several Early Paleo-zoic ophiolite belts in North Qilian and adjacent periphery Qaidam microplate imply that an archipelagic ocean during Ordovician existed in the east of Pro-Tethys.  相似文献   

4.
Ladinian radiolarian fauna, including Muelleritortis, Baumgartneria, Oertlispongus, Paroertlispongus, Pseudoertlispongus, etc., was discovered from the siliceous rock of the Runiange Formation in the Xianshuihe belt, West Sichuan Province. Geochemical test on five samples from the siliceous rock indicates that SiO2 content varies in 71.16%-90.06% and Si/Al ratio, in 49-71, which shows that the siliceous rock contains more terrigenous mud sediments. The siliceous rock is characterized by the large ratios of Al2O3/(Al2O3+Fe2O3) (0.63-0.81) and Ti/V (>26), the low ratio of V/Y (<2.8), and low vanadium content (<23 μg/g), which are similar to the geochemical characteristics of continental margin siliceous rock. The Ce/Ce* ratios of the four samples vary in 1.02-1.47 and the LaN/CeN ratio, in 0.75-1.07, which imply that the siliceous rock was deposited in the continental margin basin. But only one sample is similar to the oceanic siliceous rock in REE. Turbidite-siliceous rock bearing radiolarian-basalt asse  相似文献   

5.
The South China, including Yangzi Craton and the Cathaysian Block, belongs to the southern part of East Asia continent. It borders Pacific plate on the east side and Qinling-Dabie Orogen on the north side. During the middle-late Cretaceous, a number of downfaulted red basins (the terrestrial sedimentary basins in Fig. 1) and volcanic- sedimentary basins had been developed in South China[1,2], in which the con-temporaneous basaltic rocks were generally distributed (Fig. 1). Although the …  相似文献   

6.
We describe the mode of occurrence and geochemical characteristics of basalts, in the Khangai–Khentei belt in Mongolia, overlain by Middle Paleozoic radiolarian chert in an extensive accretionary complex. These basalts are greatly enriched in K, Ti, Fe, P, Rb, Ba, Th, and Nb in comparison to the composition of the mid‐ocean ridge basalts, indicative of within‐plate alkaline type. Ti/Y vs Nb/Y and MnO/TiO2/P2O5 ratios of the basalts also suggest within‐plate affinities. Considering the geochemical characteristics as well as the conformable relationship with the overlying radiolarian chert, the alkaline basalts were clearly not continental but formed a pelagic oceanic island. The mode of occurrence and geochemistry of the basalts show that the alkaline basaltic volcanic activity had taken place to form an oceanic island in the Paleozoic pelagic region sufficiently far from continents to allow radiolarian ooze accumulation.  相似文献   

7.
Here we report a detailed trace element study of the cherts from Liuchapo Formation, which is a terminal Ediacaran (551-542 Ma) succession in South China deposited in deep-water basinal setting. The REE of Liuchapo cherts shows similar features as observed for anoxic modern seawater (but not for hydrothermal fluids), characterized by positive La anomaly (LaN/CeN = 0.83–1.91, average 1.37), moderately negative Ce anomaly (0.53–1.1, average 0.73), positive Gd anomaly (average 1.08), positive Y anomaly (average 1.21), and depleted LREE and MREE. In addition, the Liuchapo cherts have low ΣREE (3.36–56.13 ppm, average 20.6 ppm), low Al2O3, Ti, Th and Zr concentrations, and high Y/Ho ratios (up to 43.9). The redox-sensitive trace elements concentrations in the cherts do not correlate with detrital input proxies. All of these features suggest that the redox-sensitive trace elements in the cherts were authigenically concentrated in water column and their concentrations thus are excellent indicators of ancient redox conditions. Very low Th/U ratios, high V/(V+Ni) and Fe⊤/Al ratios, enrichments of redox-sensitive trace elements (U, V, Mo), and low concentration of Mn in the cherts imply anoxia in the deep seawater. Our data reveal that the terminal Ediacaran ocean was not completely oxidized and the deep ocean was still anoxic, at least in South China. We propose that although the oxidative events existed in the terminal Ediacaran oceans, decomposition of organic matter prolonged anoxia in the deep ocean. Supported by National Natural Science Foundation of China (Grants Nos. 40532012, 40873007, 40603021) and Chinese Academy of Sciences (Grant No. KZCX3-SW-141)  相似文献   

8.
At the top of the Lower Permian Maokou Formation limestones are developed carbonaceous cherts(Plm3),which constitute the dominant seleniferous layer of the Yutangba Se deposit.The cherts contain as much Se as 1646×10?6 on average.In addition,they are rich in organic carbon,Al2O3,Si2O,but poor in S.In addition to Se,as well as Mo,Cd,V,and Co,etc are also highly enriched in the cherts.The chert samples are characterized by low ?REE,slight LREE enrichment,relatively heavy Si isotope enrichment,and insignificant variations in ? 30Si value within the range of 1.1‰-1.2‰.Generally,it can be judged from the major element,trace element and REE data and the Si isotopic characteristics that the Yutangba seleniferous cherts were formed in the shallow sea to semi-deep sea anoxic environments and their formation is controlled chiefly by bio-chemical processes.  相似文献   

9.
The stratigraphical sequences composed of chert and basalt were found in the Daxinshan area of Simao and the Manbie area of Jinghong, southwestern Yunnan. The Middle Permian to ealiest Late Permian radiolarians, such as Follicucullus and Pseudoalbaillella, have been identified from the chert. The chert from the Manbie area of Jinghong is characterized by high SiO2content (over 92%), large ratios of MnO/TiO2 (2.15) and low ratios of Al/(Al+Fe+Mn) (≤0.1) and Ce/Ce*(0.4), which indicate that the chert was deposited in pelagic basin. The chert from the Daxinshan area of Simao, however, is characterized by low SiO2 content, low ratios of MnO/TiO2 (0.27) and high ratios of Al/(Al+Fe+Mn) (0.49) and Ce/Ce*(0.88), which imply that the chert was deposited in continental margin basin. The basalts from the both areas belong to tholeiite series, and the chemical compositions of their major, rare earth and trace elements show the characteristics of MORB. These results evidence that there are volcanic rocks and chert sequences representing pelagic basin and oceanic basin near continent. These sequences and the formerly reported island-arc volcanic rock sequences imply that the Daxinshan Formation in the Lancangjiang belt represents a sedimentary assemblage formed in active continental margin basin.  相似文献   

10.
The Khoy ophiolitic complex in Northwestern Iran is a part of the Tethyan ophiolite belt, and is divided into two sections: the Eastern ophiolite in Qeshlaq and Kalavanes (Jurassic–Cretaceous) and the Western ophiolite in Barajouk, Chuchak and Hessar (Late Cretaceous). Our chromitites can be clearly classified into two groups: high‐Al chromitites (Cr# = 0.38–0.44) from the Eastern ophiolite, and high‐Cr chromitites (Cr# = 0.54–0.72) from the Western ophiolite. The chromian spinels in high‐Al chromitite include primary mineral inclusions mainly as Na‐bearing diopside and pargasite with subordinate rutile and their formation was probably related to reaction between a MORB (mid‐ocean‐ridge basalt)‐like melt with depleted harzburgite, possibly in a back‐arc setting. Their host harzburgites contain clinopyroxene with higher contents of Al2O3, Na2O, Cr2O3, and TiO2 relative to Western harzburgites and are possibly residue after moderate partial melting (~15 %) whereas the Western harzburgite is residue after high partial melting (~25 %). The chromian spinel in the Western Khoy chromitites contains inclusions such as clinopyroxene, olivine and platinum group mineral‐bearing sulfides. These Western chromitites were possibly formed at two stages during arc growth and are divided into the moderately high‐Cr# chromitites (Barajouk and Hessar) and the high‐Cr# chromitites (Chuchak A and C). The former crystallized from island‐arc‐tholeiite (IAT) melts during reaction with the host depleted harzburgites, whereas the latter crystallized from boninitic melts (second stage melt) during reaction with highly depleted harzburgite in a supra‐subduction‐zone environment. Based on the mineral chemistry of chromian spinels, pyroxenes, and mineral inclusions, the chromitites and the host peridotites from the Eastern and Western Khoy ophiolites were formed in a back‐arc basin and arc‐related setting, respectively. The Khoy ophiolitic complex is a tectonic aggregate of the two different ophiolites formed in two different tectonic settings at different ages.  相似文献   

11.
Major, trace and rare earth elements were measured in 27 samples of the Middle to Late Permian limestones from the Tieqiao section located on the marginal zone of an isolated platform (Laibin, South China). Shale-normalized REE+Y patterns of all samples show notably negative Ce anomalies (0.21–0.66, average 0.33), slightly positive Gd anomalies (1.08–1.30, average 1.20), and positive Y anomalies with superchondritic Y/Ho ratios (36–91, average 55), which are consistent with those of modern shallow seawater. Their relative LREEs enrichment with higher NdSN/YbSN ratios (0.58–1.80) than those of modern shallow seawater (0.21–0.50) suggests complicated sources of REEs for all samples. Compared with geochemical features of sediments influenced by terrigenous particles and hydrothermal fluids, it is concluded that ambient shallow seawater was the primary source of REEs in these limestones. Comparing the indicators of REE+Y elements (ΣREE, NdSN/YbSN, Ce/Ce*, Gd/Gd*, Eu/Eu* and Y/Ho) in limestones with those in bedded cherts from the Tieqiao section, we consider that limestone and bedded chert have similar sources of REE+Y elements: ambient shallow seawater with more or less hydrothermal fluids. In addition, there is a completely negative correlation between CaCO3 and SiO2 contents in limestones and bedded cherts. These results imply that precipitation of CaCO3 was inhibited by that of SiO2 which was derived mainly from hydrothermal fluid, especially in bedded cherts from the Tieqiao section.  相似文献   

12.
Four layers of cherts were found for the first time in the Yangla copper deposit, western Yunnan Province. The cherts possessed the following geochemical characteristics: 1 Low TiO2 and Al2O3 contents, but high ore-forming element (e.g. Cu, Au, Ag) contents; 2 low total REE contents and clear negative Eu anomalies when normalized to chondrite similar to the REE contents and distribution patterns of associated massive sulfide ores; 3 silicon isotopic compositions of cherts in the Yangla deposit being the same as cherts and geyserite of hot-water sedimentary origin; 4 lead and sulfur isotopic compositions of cherts in the Yangla deposit being similar to those of the massive sulfide ores in the Yangla deposit; 5 Rb-Sr isochron age of cherts from the Yangla deposit being identical with that of host strata. Hence, we conclude that the cherts in the Yangla deposit are of hot-water sedimentary origin, which have a close relationship with the massive sulfide ores. The discovery of hydrothermal cherts from the Yangla copper deposit provides further evidence for the hydrothermal exhalative origin of the massive sulfide deposits.  相似文献   

13.
Dong  YunPeng  Zhang  GuoWei  Yang  Zhao  Zhao  Xia  Ma  HaiYong  Yao  AnPing 《中国科学:地球科学(英文版)》2007,50(2):234-245

The mafic-ultramafic assemblages, which thrustthrust into the Wushan-Tangzang boundary fault as some blocks and outcropped in the Yuanyangzhen, Lijiahe, Lubangou and Gaojiahe area, consist mainly of meta-peridotites, gabbros and basalts. The meta-peridotites are characterized by high SiO2 and MgO contents, low ΣREE, as well as their chondrite-normalized rare earth element patterns show some similarities to that of middle oceanic meta-peridotite. The basalts from the Yuanyangzhen, Lijiahe and Lubangou area are characterized by relatively high TiO2 content, low Al2O3 content and Na2O>>K2O. Above all, it is the slight enrichment or flat REE distribution patterns and the unfractionated in HFS elements in the primitive-normalized trace elements distribution patterns that indicate these basalts are similar to that of the typical E-MORB. In comparison, the basalts from the Gaojiahe section are featured by depletion in Nb and Ta contents and enrichment in Th content which show that these were derived from an island-arc setting. From studies of the regional geology, petrology, geochemistry, geo-chronology and all above evidence, it can be suggested that the mafic-ultramafic rocks from the Wushan area are mainly dismembered E-MORB type ophiolite, which represent the fragments of the lithosphere of the Early-Paleozoic Qinling ocean. It is preferred that these rocks were formed in an initial mid-ocean ridge setting during the beginning stage of the oceanic basin spreading. This ophiolite together with the Gaojiahe island-arc basalts shows that there exists an ophiolitic mélange along the Wushan-Tangzang boundary fault, and marks the suture zone after the closure of the Qinling ocean in early Paleozoic.

  相似文献   

14.
CO2 fluid inclusions in mantle minerals are an im-portant source for us to get the information of mantle fluids. Fluid inclusions are mainly composed of CO2, with minor CO, H2O, CH4, N2, H2S, SO2, F, etc., which were demonstrated by lots of Raman spec-trometer analyses in recent years. In contrast, there are very few researches on CO2-bearing melt inclusions since it is more difficult to do so. The available studies have found that the primary CO2-bearing melt inclu-sions are basaltic …  相似文献   

15.
The succession recovered in ODP hole 107–651 in the young oceanic Vavilov basin (Tyrrhenian Sea) comprises, beneath a thick Pleistocene to Upper Pliocene sedimentary cover (chiefly volcanoclastics), four basement units: (1) MORB-type basaltic pillows and breccias; (2) a complex succession made of dolerites, albitites, basaltic breccias, metadolerite pebbles (including an intercalated sandy layer with periodotite clasts); (3) MORB-type basaltic pillows and breccias; (4) highly serpentinized peridotite. Between units 3 and 4, granitoid pebbles occur.This sequence is surprisingly similar to successions known in the Western Alps' Tethyan ophiolites. There, the sediments (Callovian-Oxfordian radiolarian cherts) lie stratigraphically upon breccias mostly derived from underlying serpentinite, and sometimes gabbroic basement. At some places, thin basaltic (tholeiitic) pillows and breccias occur between the radiolarian cherts and the breccias.From the comparison between a present day setting (the central Tyrrhenian Sea) and a formerly emplaced basement succession (the Western Alps), we stress the following (a) both the here-discussed ophiolites and oceanic basement are different from classical ophiolite sequences; (b) both occurrences imply unroofing of mantle rocks that therefore were directly outcropping on the seafloor; (c) such a comparison may indicate a very slow spreading rate for the Alpine Tethyan ocean.  相似文献   

16.
We discuss the chemical compositions of rhyolites from three distinct tectonic settings: (i) the continental rift from Ethiopia (both Oligocene–Miocene and Quaternary rhyolites); (ii) the early Miocene continental arc of Japan (the Mt Wasso rhyolites related to the rifting of the Japan Sea); and (iii) the oceanic Izu–Bonin Island Arc. The comparison reveals that the oceanic island arc rhyolites have high contents of CaO, Al2O3, and Sr, and extremely low abundance of trace elements including K2O. In contrast, the Ethiopian continental rift rhyolites are characterized by low contents of CaO, Al2O3, and Sr, and high contents of K2O, and are enriched in the whole range of trace elements. The continental arc Mt Wasso rhyolites are apparently low in Nb content, although they display similar chemical trends to those of the Ethiopian rhyolites. This obvious difference in the chemical signatures of the rhyolites from the three tectonic settings is the consequence of their derivation from different sources. The implication of this result is that fractional crystallization processes were dominant in the rift‐related rhyolites both from continental rift and continental arc regardless of the prevailing tectonic setting and the nature of the crust (age, thickness, composition), whereas the oceanic island arc rhyolites may form through partial melting of young, mafic crust.  相似文献   

17.
Abstract This paper contains extended abstracts of the seven papers presented at the symposium 'Radiolarians and Orogenic Belts' held at the seventh meeting of the International Association of Radiolarian Paleontologists (INTERRAD). Important results of the symposium include the following: (1) Upper Paleozoic and Mesozoic cherts are widely distributed within accretionary complexes in the circum-Pacific orogenic belt. Radiolarian dating reveals that long durations of chert sedimentation in a pelagic environment are recorded on both sides of Pacific-rim accretionary complexes (e.g. New Zealand, Japan, Russian Far East, Canadian Cordillera). (2) Triassic radiolarian faunas from New Zealand and the Omolon Massif, northeast Siberia are similar in composition and are characterized by the absence of typical Tethyan elements. This suggests that radiolarian faunal provincialism may have been established as early as the Triassic. High-latitude radiolarian taxa exhibit a bi-polar distribution pattern. (3) The Lower Triassic interval in chert dominant pelagic sequences is mechanically weaker than other levels and acted as a décollement in accretionary events. This lithologic. contrast in physical property is considered to reflect radiolarian evolution, such as the end-Permian mass extinction.  相似文献   

18.
This study is focused on a plagioclase‐bearing spinel lherzolite from Chah Loqeh area in the Neo‐Tethyan Ashin ophiolite. It is exposed along the west of left‐lateral strike‐slip Dorouneh Fault in the northwest of Central‐East Iranian Microcontinent. Mineral chemistry (Mg#olivine < ~ 90, Cr#clinopyroxene < ~ 0.2, Cr#spinel < ~ 0.5, Al2O3orthopyroxene > ~ 2.5 wt%, Al2O3clinopyroxene > ~ 4.5 wt%, Al2O3spinel > ~ 41.5 wt%, Na2Oclinopyroxene > ~ 0.11 wt%, and TiO2clinopyroxene > ~ 0.04 wt%) confirms Ashin lherzolite was originally a mid‐oceanic ridge peridotite with low degrees of partial melting at spinel‐peridotite facies in a lithospheric mantle level. However, some Ashin lherzolites record mantle upwelling and tectonic exhumation at plagioclase‐peridotite facies during oceanic extension and diapiric motion of mantle along Nain‐Baft suture zone. This mantle upwelling is evidenced by some modifications in the modal composition (i.e. subsolidus recrystallization of plagioclase and olivine between pyroxene and spinel) and mineral chemistry (e.g. increase in TiO2 and Na2O of clinopyroxene, and TiO2 and Cr# of spinel and decrease in Mg# of olivine), as a consequence of decompression during a progressive upwelling of mantle. Previous geochronological and geochemical data and increasing the depth of subsolidus plagioclase formation at plagioclase‐peridotite facies from Nain ophiolite (~ 16 km) to Ashin ophiolite (~ 35 km) suggest a south to north closure for the Nain‐Baft oceanic crust in the northwest of Central‐East Iranian Microcontinent.  相似文献   

19.
Major element, trace element and Sr, Nd, Pb and O isotopic data for a Franciscan Mn-deposit suggest an origin by seafloor hydrothermal circulation. Based onQ-mode factor analysis the cherts and Mn-lenses of the Blue Jay mine formed from a combination of 4 components representing 1 biogenic, 1 hydrothermal, and 2 detrital sources. RbSr, UThPb and O isotopic systematics in the Mn-lenses were affected by input from the hydrothermal circulation of material leached from the underlying basalts. Nd isotopic compositions in both cherts and Mn-lenses are identical and within the range measured for Pacific Ocean water suggesting the REE were not mobilized by hydrothermal activity. Correlation of δ18O with SiO2 and MnO2 in the Mn-lenses implies the lenses formed by simple mixing of hydrothermally derived Mn-oxides with seawater and biogenic silica. δ18O of the cherts is both uniform and depleted relative to DSDP Jurassic cherts but similar to microquartz-bearing cherts of the Monterey Formation: this suggests that diagenetic activity exerted more control on oxygen isotope compositions then hydrothermal alteration or metamorphism. Finally, a well defined RbSr isochron of158 ± 5Myr was obtained for these cherts and opens the possibility of determining absolute radiometric ages for similar cherts throughout the geologic record.  相似文献   

20.
Abstract The abundance of magnetic microspherules in a Triassic-Jurassic continuous sequence of alternating chert and shale beds in the Mino accretionary complex, central Japan, was measured systematically. Depending on time, the magnetic microspherules extracted from shale beds change in abundance considerably from the minimum 0.9ppm/cm3 at latest Triassic ( ca 208Ma) and the maximum 75ppm/cm3 at late Early Jurassic ( ca 187Ma); however, the abundance is always higher approximately 10–100 (average 70) times than those from adjacent chert bed at any stratigraphic horizon. Such systematic difference reveals the origin of radiolarian bedded chert as cyclic-rapid accumulation of biogenic SiO2 under extremely slow accumulative environments of shale with probable aeolian dust in origin. The accumulation data for individual shale and chert beds were obtained based on the microspherule abundance and radiolarian biostratigraphy, i.e., ca 0.018g/cm2Ka for lower Jurassic shale beds and ca 1.9g/cm2Ka for adjacent chert beds.
Duration time to make a chert-shale couplet corresponds to a dominantly 15–20Ka interval (average 23 Ka) in Upper Triassic bedded cherts with a low paleolatitude, whereas a 40–45 Ka interval (average 42 Ka) in Lower Jurassic ones which may been formed in higher latitude than Triassics before the final accretion to the Asian continental margin. Depending on paleolatitude, the cyclicity of 23 and 42 Ka may correspond to Milankovitch cycles which have been well documented in deep-sea sediments.  相似文献   

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